This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Lithography followed by plasma etching is the standard method for manufacturing microelectronics and many other devices. Requirements on lateral resolution and vertical dimensions translate into significant engineering challenges for the lithographic imaging layer (resist).
Photolithography and electron-beam lithography are the most successful and commonly used lithography techniques for preparing high-resolution patterns. The success of these methods is highly dependent on the processing of the resist layer. A resist material needs to satisfy requirements on resolution, exposure sensitivity, contrast, line-edge roughness, depth-of-focus, and etch resistance. To satisfy the requirements of both resolution and line-edge roughness, the resist layer must be thin. For photolithography, high resolution is achieved by reducing the wavelength of the light source and increasing the numerical aperture. The trade-off for improving resolution using this method is the reduction in the depth-of-focus. This imposes a fundamental upper limit on the resist thickness. In the case of electron-beam lithography, forward scattering of electrons is a fundamental limitation on resolution. Scattering is minimized by using a thin resist layer, in addition to increasing the electron energy. Thus, in both techniques, as a result of different physical limitations, the thickness of the resist layer suitable for high-resolution patterning is limited. Issues associated with pattern collapse also effectively limit the thickness of resist films.
Significantly, the limit on the resist layer thickness runs contrary to the needs of pattern transfer to a substrate by plasma etching or ion milling. Most resist materials erode significantly and fairly quickly relative to the substrate during plasma etching or milling. If the resist layer is too thin, then the pattern cannot be etched deeply into the substrate using solely the resist layer as the etch mask. Accordingly, the thinness of lithography resists presently limits the fabrication of high aspect-ratio structures. To overcome this problem, the resist pattern is usually transferred to a hard mask layer that provides greater etch resistance. However, this complicates the fabrication process and leads to additional image blur, feature bias, and line edge roughness. Disadvantages associated with introducing hard masks have motivated recent work to improve the etch resistance of high resolution e-beam resists, such as poly(methyl methacrylate) (PMMA) and ZEP520A, by processing following resist exposure and development.
Accordingly, there is a need for resist materials that are resistant to etching process conditions. Greater etch resistance allows for adequate etching time necessary to create features, including high-aspect ratio features, in substrate materials such as silicon. Additionally, the resist material overlaying the substrate during plasma etching or milling should be thin so that fine pattern features can be sharply resolved in the substrate. Thus, the resist needs to have high resolution, little line-edge roughness, high resistance to plasma etching, and significant mechanical stiffness to prevent pattern collapse during wet development. Presently, no resist material satisfies all these requirements simultaneously. Such improved mask materials would be applicable to a range of microelectronic and MEMS manufacturing applications among others.
There have been attempts to improve resist etch resistance by infusing the resist film or mask with inorganic materials, most notably silicon. However, these processes suffer from problems related to line-edge roughness, excessive swelling and low contrast. Few of these methods have been demonstrated for nanoscale patterns (<100 nm). Some recent work of this area concerned photoresist materials that are not currently used in industrial production. As a result, there is a need to develop improved methods to prepare resist masks with improved etch resistance.